JP2007039732A - High strength steel component for machine structure having excellent fatigue characteristic, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength steel components for machine structure which are quench heat-treated by carburizing, nitriding, etc., and in which the additive quantities of expensive hardenability-improving elements, such as Cr, Mo and Ni, can be minimized and hereby excellent workability can be provided in a state before heat treatment and excellent fatigue characteristics can be exhibited after quenching/tempering. <P>SOLUTION: The high strength steel component for machine structure with excellent fatigue characteristics has a composition in which respective contents of C, Si, Mn, B, N, etc., are specified and at least one element selected from the group consisting of Nb, Ti, Zr, Ta and Hf is contained within the range satisfying the relation in inequality SM≥1.0×10<SP>-5</SP>(where SM satisfies equation SM=[Nb]/92.9+[Ti]/47.9+[Zr]/91.2+[Ta]/181+[Hf]/178). In the above equation, [Nb], [Ti], [Zr], [Ta] and [Hf] represent their respective amounts (mass%) in the form of solid solution which are measured from the respective extraction residues of the elements contained in the steel parts. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel part for machine structure used by carrying out a heat treatment such as carburizing treatment in a transport machine such as an automobile, a construction machine or other industrial machine, and a manufacturing method thereof, in particular, a bearing, a pulley for CVT, a shaft, etc. When used as a material for gears, gears with shafts, etc., steel parts for mechanical structures exhibiting excellent workability in the state before carburizing treatment and high strength and excellent fatigue characteristics after carburizing heat treatment and their manufacture It is about the method.

  Surface hardening heat treatment (case hardening) such as carburizing, nitriding, and carbonitriding has been conventionally applied to steel parts that require particularly high strength in parts for machine structures used for automobiles, construction machinery, and other various industrial machines. Has been done. For these applications, the case hardening steels defined by JIS standards such as SCr, SCM, SNCM, etc. are usually used. After forming into the desired part shape by machining such as forging and cutting, carburizing and carbonitriding It is manufactured through a finishing process such as polishing.

  By the way, especially in steel parts for machine structures that require high strength, in order to satisfy the required strength as a product, a large amount of hardenability improving elements such as Cr and Mo are added to increase the strength after quenching. (Patent Document 1, etc.).

  Further, in Patent Document 1, by adding a large amount of Ni that has an effect of enhancing the hardenability of steel without generating grain boundary oxides and further strengthening the dough of the carburized layer and the non-carburized layer. Increased strength. However, the above-mentioned elements added for the purpose of improving hardenability are generally expensive, and in addition to the economic difficulty of causing a large increase in material cost, the steel material before heat treatment is increased by increasing the amount of these strengthening elements. Hardening causes a practical problem that workability and machinability deteriorate.

  Therefore, by suppressing the addition amount of a hardenability improving element such as Cr, Mo, Ni, etc., while achieving cost reduction, it has excellent workability (forgeability, rollability, etc.) in the state before heat treatment, In addition, it is desired to develop a steel part for machine structure that can exhibit a high level of strength and fatigue characteristics by quenching treatment.

On the other hand, by adding trace elements such as Nb and Ti for case hardening steel, the carbides and nitrides of these elements are finely precipitated to prevent austenite (γ) crystal grains from coarsening during heat treatment. Techniques for preventing deterioration of toughness such as a weld heat affected zone are known (Patent Documents 2, 3, etc.). Such a technical idea is also described in the above-mentioned Patent Document 1. However, to the best of the present inventors' knowledge, including these patent documents, there is no example in which the trace elements as described above are used for improving the hardenability of steel for machine structural use.
JP-A-2-170944 Japanese Patent No. 3510506 JP-A-9-78184

  The present invention has been made by paying attention to the above-described circumstances, and its purpose is to provide an expensive hardenability such as Cr, Mo, Ni, etc. in steel parts for mechanical structures subjected to quenching heat treatment such as carburizing and nitriding. By reducing the addition amount of the improving element as much as possible, it has excellent workability such as hot or cold forging before heat treatment, excellent hardenability, and excellent fatigue properties after quenching and tempering. An object of the present invention is to provide a steel part for mechanical structure that exhibits high strength and to provide a useful method for producing such a steel part.

The high-strength mechanical structural steel of the present invention that has solved the above-mentioned problems is mass%, C: 0.10 to 0.4%, Si: 0.05 to 1.5%, Mn: 0.00. 3 to 3.0%, Mo: 0.5% or less (not including 0%), B: 0.0003 to 0.015%, N: 0.02% or less (not including 0%), In addition, the present invention contains at least one element selected from the group consisting of Nb, Ti, Zr, Ta, and Hf within a range of 0.50% or less and satisfying the relationship of the following formula (1). There is.
SM ≧ 1.0 × 10 ⁻⁵ (1)
However, SM = [Nb] /92.9+ [Ti] /47.9+ [Zr] /91.2+ [Ta] / 181 + [Hf] / 178
{Wherein [Nb], [Ti], [Zr], [Ta], [Hf] represent the amount of solid solution (mass%) measured from the extraction residue of each element contained in the steel part}.

  In the steel part for high-strength mechanical structure of the present invention, in addition to the above-mentioned essential elements, one or more elements selected from the groups shown in the following a) to c) are contained according to the required part characteristics. Is also effective.

a) From the group consisting of Ni: 2.0% or less (not including 0%), Cu: 2.0% or less (not including 0%), Cr: 3.0% or less (not including 0%) At least one element selected,
b) V: 0.1% or less (excluding 0%),
c) From the group consisting of Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), REM: 0.020% or less (not including 0%) At least one element selected.

  Further, the production method of the present invention is an invention positioned as a method for producing a steel part for mechanical structure having the above characteristics, and a steel material satisfying the requirements of the above chemical components is soaked at a temperature of 1250 ° C. or higher, It is characterized in that the carburizing heat treatment is performed at a temperature of 1000 ° C. or higher.

  According to the present invention, the chemical component of steel is specified, and in particular, Mo known as a hardenability improving element is used as an essential element. However, the amount of addition of Mo is minimized and a small amount of B is reduced. , Nb, Ti, Zr, Ta, Hf is supplemented by adding a trace amount of at least one element selected from the group consisting of Nb, Ti, Zr, Ta, and Hf, and has excellent workability before heat treatment and extremely hardenability. Mechanical structural steel parts that exhibit excellent strength and fatigue properties by heat treatment can be provided at a cost that satisfies the demands of customers sufficiently in terms of price.

  Under the prior art as described above, the present inventors have good hardenability, mechanical properties having excellent fatigue characteristics and high strength, even when the hardenability improving elements such as Cr, Mo, Ni are reduced as much as possible. We have been studying various additive elements in preparation for the development of hardenable mechanical structural steel parts that can obtain parts and are relatively soft and can exhibit excellent workability before quenching heat treatment. It was.

  As a result, Mo is essentially used as a hardenable element, but the amount thereof is suppressed as much as possible, and a small amount of B together with the Mo, and also Nb, Ti, Zr, Ta, and Hf called so-called microalloys. If a small amount of at least one element selected from the group consisting of the above is added, it is relatively soft and excellent workability before quenching heat treatment, and it has good quenchability and the strength after quenching is dramatic. As a result, it was found that a mechanical structural part exhibiting strength superior to conventional mechanical structural steel and excellent fatigue characteristics can be obtained, and the present invention has been completed.

  Hereinafter, the reason why the chemical components of steel parts for machine structural use are defined in the present invention will be clarified.

C: 0.10 to 0.4%;
C is an important element for securing the core hardness required as a machine structural part. If it is less than 0.10%, the static strength as a machine structural part becomes insufficient due to insufficient hardness. However, if the amount of C is too large, it becomes too hard and the toughness of the core part is deteriorated, and the workability before quenching heat treatment is also deteriorated. The C content is more preferably 0.15% or more and 0.3% or less, further preferably 0.17% or more and 0.25% or less.

Si: 0.05 to 1.5%;
Si acts as a deoxidizer and has the effect of reducing the amount of oxide inclusions and improving the internal quality, and is effective in securing the surface layer hardness after quenching by suppressing the decrease in hardness due to tempering. It is an element, and 0.05% or more of addition is required to effectively exhibit these effects. However, if the amount of Si is too much, the steel material becomes too hard and the workability is deteriorated, and the fatigue properties are deteriorated by promoting the formation of a grain boundary oxide layer during the quenching process, so the upper limit is set to 1.5%. . A more preferable Si content is 0.10% or more and 1.0% or less, and further preferably 0.2% or more and 0.8% or less.

Mn: 0.3-3.0%;
Mn acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving the internal quality of steel, and is an element indispensable for improving the hardenability. In order to make it contain, it must contain 0.3% or more. However, if Mn is too much, workability such as hot forging deteriorates, so it should be controlled not to exceed 3.0% at most. The more preferable content of Mn is 0.5% or more and 2.0% or less, more preferably 0.75% or more and 1.5% or less.

Mo: 0.5% or less (excluding 0%);
Mo is dissolved in steel and has the effect of remarkably improving hardenability by combining with solid solution B and solid solution Nb, Ti, Zr, Hf, Ta, which will be described later, the strength of carburizing / nitriding part and core part, Although it is an important element having an effect of increasing toughness, if it is too much, the hardness after hot working becomes too high and the cold workability is remarkably lowered, so 0.5% is made the upper limit. The upper limit with preferable Mo is 0.3%, More preferably, it is 0.2% or less. Although there is no particular lower limit, it is preferably 0.03% or more, more preferably 0.08% or more.

B: 0.0003 to 0.015%;
B is an element indispensable for enhancing the hardenability and toughness without deteriorating hot or cold workability. As in the present invention, B is solid solution Mo and Nb, Ti, Zr, By combining with an element selected from Ta and Hf, the hardenability is remarkably improved, contributing to high strength. In order to exert such an effect effectively, B must be contained in an amount of 0.0003% or more, preferably 0.0006% or more, and more preferably 0.0012% or more. However, the effect is saturated at 0.015%, so addition beyond that is economically wasteful. A more preferable upper limit of the B content is 0.005%, and further preferably 0.0035% or less.

N: 0.02% or less (excluding 0%);
N is an impurity element which is inevitably mixed in the steel melting process, and is combined with Nb, Ti, Zr, Ta, and Hf, which will be described later, and becomes a nitride-based coarse inclusion and deteriorates fatigue characteristics. It is better to keep it as small as possible, and at most 0.02% or less, preferably 0.01% or less, more preferably 0.007% or less.

At least one element selected from the group consisting of Nb, Ti, Zr, Ta, and Hf: each 0.50% or less, and the formula (1), that is, “SM ≧ 1.0 × 10 ⁻⁵ ” The range that satisfies the relationship;
In the present invention, it is an essential requirement to control the contents of the above five elements to 0.50% or less and to control them in the range of “SM ≧ 1.0 × 10 ⁻⁵ ”.

  Here, “SM” means “[Nb] /92.7+ [Ti] /47.9+ [Zr] /91.2+ [Ta] / 181 + [Hf] / 178”, that is, extraction of quenched and tempered steel parts. This is the total amount of solid solution of each element obtained by residue analysis and precipitates such as carbides and nitrides. The amount of solid solution is calculated by the chemical analysis of the residue obtained by electrolytic extraction and the difference between the added amount and the precipitate. The precipitate is collected because it is collected using a 0.1 μm filter. The thing and several nanometer level precipitate pass through a filter. Therefore, the amount of solid solution here is the sum of the amount actually dissolved and the fine precipitates on the nm level.

Therefore, a large SM value means that the amount of the solid solution and the amount of fine precipitates are large. According to the present inventors, the fatigue characteristics increase as the SM value increases. It was found that those having a value of 1 × 10 ⁻⁵ or more show high fatigue properties, whereas those having a value of less than 1 × 10 ⁻⁵ are poor. The reason is that not only the solid solution Nb, Ti, Zr, Ta, Hf itself contributes to the improvement of the fatigue characteristics, but also the fine precipitates precipitated during tempering contribute to the further improvement of the fatigue characteristics. Conceivable. In order to exhibit stable fatigue characteristics at a higher level, the SM value should be 2 × 10 ⁻⁵ or more, and a more preferable SM value is 4 × 10 ⁻⁵ or more.

  Of the five elements described above, Ti has a remarkable effect of fixing N and promoting the solid solution of B. Therefore, Ti is preferably contained in an amount of 0.01% or more, more preferably 0.03% or more. However, if it exceeds 0.5%, the deposited nitride becomes coarse and adversely affects the fatigue characteristics, so at most 0.5% or less, preferably 0.1% or less, more preferably 0.05. It is good to keep it below%.

  On the other hand, since Nb does not generate coarse nitrides, 0.5% or more may be contained. However, the effect of improving the fatigue characteristics is not increased any more, and it is economically disadvantageous. Therefore, it should be suppressed to 0.5% or less, more preferably 0.1% or less, and still more preferably 0.05% or less. It is. Further, if too much Zr, Ta, Hf is formed, coarse nitrides are formed and the fatigue characteristics are impaired, so at most 0.5% or less, preferably 0.1% or less, more preferably 0.05% or less. It is good to suppress.

  From the viewpoint of improving hardenability, the effects of the above five types of elements are almost saturated at a total of about 0.5%, and when it exceeds this level, the formation of coarse carbides adversely affects machinability and fatigue characteristics. Therefore, the total sum should be suppressed to about 0.5% or less, more preferably 0.20% or less.

  The essential constituent elements of steel constituting the component of the present invention are as described above, and the balance is substantially Fe. “Substantially” means that inevitable mixing of elements inevitably mixed, such as Al, P (phosphorus), O (oxygen), and S (sulfur), is allowed. In order to suppress obstacles due to inclusion as much as possible, Al is preferably 0.5% or less, P is 0.05 or less, O is 0.003% or less, and S is 0.1% or less.

Incidentally, Al generates hard and coarse non-metallic inclusions (Al ₂ O ₃ ) and deteriorates impact characteristics and cold workability, so it is suppressed to 0.5% or less, more preferably 0.2% or less. Should. P segregates at the grain boundaries and lowers impact resistance and cold workability. Therefore, P should be suppressed as much as possible, and at most 0.05% or less, more preferably 0.03% or less is good. . Further, O (oxygen) lowers the strength characteristics of the steel material, so 0.003% or less, more preferably 0.0015% or less is preferable. While S adversely affects toughness, MnS is formed and contributes to the improvement of machinability. Therefore, when machinability is required, addition of an appropriate amount is effective. However, in order to avoid toughness deterioration, it is good to restrain to 0.1% or less, More preferably, 0.08% or less, More preferably, 0.02% or less.

  In addition to the above essential elements and unavoidable elements, the steel material used in the present invention is effective to contain the following selective elements in order to give additional additional characteristics as desired. Those added with these elements are also included in the technical scope of the present invention.

At least one selected from Ni: 2.0% or less, Cu: 2.0% or less, Cr: 3.0% or less;
Each element of Ni, Cu, and Cr has an effect of increasing the strength and toughness of the carburized portion and the core portion, and one or two or more types are suitably used (preferably, each is 0.1. About 1% or more) is effective. However, if too much, the hardness after hot working becomes too high and cold workability deteriorates, so Ni is 2.0% or less, Cu is 2.0% or less, Cr: 3.0% or less, More preferably, each of them is 1.5% or less, and further preferably, each of 1.2% or less. Among these, Cr has an excellent effect of improving hardenability, so it is preferably 0.2% or more, more preferably 0.5% or more.

V: 0.1% or less;
V has the effect of increasing the hardenability and increasing the resistance to temper softening when added in a small amount, and the effect is effectively exhibited at about 0.005% or more. However, if too much, cold workability deteriorates, so it should be suppressed to 0.1% or less. More preferably it is 0.05% or less, and still more preferably 0.02% or less.

At least one selected from Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.02% or less;
All of these elements react with S in steel to form sulfides and prevent MnS from stretching, thereby improving toughness and fatigue properties, and also effectively improving machinability. To do. However, if too much, on the contrary, the toughness is remarkably deteriorated, so even if it is added, it should be kept below the upper limit value. More preferable upper limit values are Ca: 0.003%, Mg: 0.003%, and REM: 0.01%.

  Next, in order to obtain a mechanical structural steel part having the above-described characteristics, a steel material that satisfies the above-mentioned chemical composition requirements is soaked at a temperature of 1250 ° C. or higher and carburized and quenched (case hardening). It is important to set the heating temperature to 1000 ° C. or higher.

  First, setting the soaking temperature to 1250 ° C. or more promotes the solid solution of Nb, Ti, Zr, Ta, Hf during carburizing and quenching, and exhibits the effect of increasing the SM value after carburizing and quenching. This is important for suppressing crystal grain growth during quenching and suppressing deterioration of core toughness. That is, once carbides such as Nb, Ti, Zr, Ta, and Hf are once solid-dissolved, they can be easily re-dissolved during carburizing heating, and can promote fine and uniform precipitation and suppress the growth of crystal grains. is there. A more preferable soaking temperature for exhibiting such an effect is around 1300 ° C.

  Further, the heating temperature during carburizing is set to 1000 ° C. or higher. When the carburizing temperature is increased in this manner, solid solution of Nb, Ti, Zr, Ta, and Hf is promoted to increase the SM value and improve the fatigue characteristics. At the same time, the quenching effect is enhanced and the static strength and fatigue strength of the parts are improved. From such a viewpoint, a more preferable carburizing heating temperature is 1050 ° C. or higher, more preferably 1100 ° C. or higher.

  Thus, according to the present invention, the chemical component is specified, and in particular, by adding a trace amount of at least one selected from Nb, Ti, Zr, Ta, and Hf together with B, it has excellent workability before quenching treatment. After carburizing and quenching, it is possible to provide steel parts for machine structures having a high level of strength and fatigue characteristics.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

Example 1
Steel materials having chemical compositions shown in Tables 1 and 2 were melted in a small smelting furnace, and after casting and soaking, hot forging was performed to obtain a steel bar having a side of 155 mm square. Using this steel bar, as shown in Table 3, it was soaked at a temperature of 1100 to 1300 ° C. for 60 minutes and then air-cooled. After that, by reheating to 950-1100 ° C. and rolling at a final rolling temperature of 850-950 ° C., a round steel bar having a diameter of 30 mm is obtained, and a cylindrical specimen having a diameter of 12 mm × height of 18 mm is created from this round steel bar, A hardness test was performed by the following method, and quenching characteristics and crystal grain growth characteristics were examined.

[Hardening characteristics, crystal grain growth characteristics, core hardness]
After cold forging each test bar steel at a reduction rate of 70%, crystal grains were measured for those subjected to carburizing treatment and oil quenching at 1050 ° C. for 3 hours. In addition, a JIS No. 3 Charpy impact test piece was prepared for each of the 30 mm diameter round steel bars obtained above and subjected to carburizing treatment + oil quenching at 1050 ° C. for 3 hours and then tempering at 160 ° C. Tests were conducted to measure core toughness. Moreover, the Vickers hardness of each test piece core part was measured by 10 kg of loads. The core toughness was 20 J or more and the core hardness was Hv330 or more.

[Crystal grain suppression effect]
According to the austenite grain size test method defined in JIS G 0551, the area ratio of coarse grains having a grain size number of 5 or less was measured for the γ grain size of the sample cross section obtained after the carburizing treatment obtained above. Those exceeding 20% were judged as bad (x) and those below 5% as acceptable (O).

[Electrolytic extraction residue and solid solution amount]
A 10% acetylacetone-1% tetramethylammonium chloride-methanol solution is used as an electrolytic solution, and electrolytic extraction is performed at room temperature at a current of 200 A / m ² or less, and the extraction residue is filtered through a 0.1 μm filter. And the difference of the total addition amount and the amount in a residue was calculated | required, and it was set as the amount of solid solution.

[Rolling fatigue life]
Using the rolling fatigue test piece (diameter 60 mm × thickness 5 mm disk-shaped test piece) shown in FIG. 1 and lapping the surface roughness to an average roughness of 0.04 μmRa or less, the following conditions A rolling fatigue test was conducted to determine the life (L10) when the failure rate was 10%, and 3 × 10 ⁶ or more was accepted.

Rolling fatigue test conditions; surface pressure: 527 kg / mm ² , rotation speed: 1000 rpm, number of steel balls: 6, lubrication oil: trade name “turbine oil # 68” manufactured by Nippon Oil Corporation, number of tests (n): 10 times

  The results are collectively shown in Table 3.

  From Tables 1 to 3, it can be considered as follows.

  No. 2, 3, 5 to 18 are examples that satisfy all the requirements of the present invention, soaking temperature, quenching temperature is appropriate, SM value is also suitable, abnormal growth of γ crystal grains is small, core toughness, Good core hardness and excellent rolling fatigue characteristics.

  No. No. 1 has a low SM value because the quenching temperature is too low, and has poor rolling fatigue characteristics. No. No. 4 has a low SM value because the soaking temperature is too low, abnormal growth of γ crystal grains occurs, and the rolling fatigue characteristics are somewhat poor. No. Nos. 19 and 20 are comparative examples in which the steel material components deviate from the prescribed requirements. No. 19 lacks B content. No. 20 has a poor amount of C, and therefore has poor rolling fatigue characteristics.

It is explanatory drawing which shows the test piece for a rolling fatigue test employ | adopted by experiment.

Claims (5)

C: 0.10 to 0.4% (in the case of chemical components, means mass%, the same shall apply hereinafter),
Si: 0.05 to 1.5%,
Mn: 0.3-3.0%
Mo: 0.5% or less (excluding 0%),
B: 0.0003 to 0.015%,
N: 0.02% or less (excluding 0%),
And at least one element selected from the group consisting of Nb, Ti, Zr, Ta, and Hf is contained in a range that is 0.50% or less and satisfies the relationship of the following formula (1). High-strength mechanical structural steel parts with excellent fatigue characteristics.
SM ≧ 1.0 × 10 ⁻⁵ (1)
However, SM = [Nb] /92.9+ [Ti] /47.9+ [Zr] /91.2+ [Ta] / 181 + [Hf] / 178
{Wherein [Nb], [Ti], [Zr], [Ta], [Hf] represent the amount of solid solution (mass%) measured from the extraction residue of each element contained in the steel part}.   Steel has other elements as follows: Ni: 2.0% or less (not including 0%), Cu: 2.0% or less (not including 0%), Cr: 3.0% or less (including 0%) The mechanical structural steel part according to claim 1, comprising at least one element selected from the group consisting of:   The steel part for machine structure according to claim 1 or 2, wherein the steel further contains V: 0.1% or less (not including 0%) as another element.   Still other elements of steel are: Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), REM: 0.02% or less (0% The steel part for machine structure according to any one of claims 1 to 3, which contains at least one element selected from the group consisting of:   A steel material that satisfies the chemical component requirements described in any one of claims 1 to 4 is soaked at a temperature of 1250 ° C or higher, and is carburized and heated at a temperature of 1000 ° C or higher and has excellent fatigue characteristics. Of high strength steel parts for mechanical structures.

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